1. Field
This application relates generally to systems and methods for calibrating and fitting cochlear implants.
2. Related Art
A cochlear implant is a device that may be surgically implanted into a person's cochlea to stimulate it to cause hearing. It may include a tiny receiver which is placed under the skin in the bony part behind the ear. The receiver may have a probe with several electrodes that is implanted into the cochlea. A cochlear implant can be useful if the hearing loss is caused by problems with the cochlea (usually damaged hair cells) or where the loss is caused by problems with the middle ear that cannot be corrected.
A person with a cochlear implant may also wear a hearing aid like device that has a microphone, a processor and a transducer. The processor may manipulate what the microphone hears and may send a signal to the transducer, which may be worn just behind the ear. The transducer may change the signal from an electrical signal to a magnetic signal that can be received through the skin by the implanted receiver. The receiver may then stimulate the probe in the cochlea causing the person to hear.
Great variability in preferences for and performance with various speech processing strategies in cochlear implant patients may arise from inherent and/or learned differences in the way that individuals interpret conflicting cues about pitch. Modern cochlear implants support multiple speech processing strategies (such as the CLARION family of implants from Advanced Bionics Corp.) in order to improve the chances of finding one that provides satisfactory hearing for the patient. Each strategy may require multiple fitting parameters to be determined through interactive psychophysical tests. The whole fitting process can be difficult and tiresome for the patient and time-consuming to the audiologist who adjusts the parameters and administers the tests. It may not even be immediately obvious which strategy works best for the patient. This may instead require a prolonged period of use and learning with each strategy to obtain a meaningful comparison.
As cochlear implant technology has developed, the numbers and ranges of parameters that may be set have increased greatly. The CLARION II implant has 16 independently and simultaneously programmable current sources whose output levels can be changed in less than 2 μs. Its external speech processor can digitally filter sound input into 32 or more frequency bands, each of which can be mapped to virtually any combination of electrodes and stimulus waveforms. Sounds can be represented as simultaneous analog waveforms, interleaved biphasic waveforms, or various combinations of overlapping waveforms called “virtual channels”. The repetition rate of pulsatile stimulation can exceed 20 kpps on each electrode. Any and all of these parameters may control the nature of the sound perception in patients, but often in ways that are unpredictable and difficult to correlate with their preferences and performance with particular speech processing strategies. An exhaustive search of the parameter space may be impractical.
The growing heterogeneity of ad hoc strategies and cognitive performance suggests that electrical stimulation may produce un-physiological effects on a neural mechanism for pitch perception that is different from the place-pitch and rate-pitch mechanisms on which cochlear implants have been based (see below). This may result in perceptual dissonance that subjects resolve in different ways. Until that unknown mechanism is identified and controlled, the evolution of cochlear implants may be frustratingly empirical and fragmented. Indeed, recent tests of new and “improved” strategies based on classical theories of auditory perception can confound predictions, and may produce inconsistent results at best.